This invention relates to a photometric device for controlling automatic exposure in an endoscope using a fibre optical system, such as optical glass fibre, and more particularly, to a photometric device which is effective to control the illumination intensity of the endoscope lamp without substantially interfering with the image light.
As is well known, an endoscope is a device having a tube which is designed for insertion into a human cavity, an objective lens at the tip of the tube, illuminating and viewing means in a housing at the opposite end of the tube, and fibre optical transmission means for carrying illuminating light from the housing to the tip and for carrying the image reflective light from the tip back to the housing for viewing by a physician or the like. Furthermore, it is known for such endoscopes to include special lights for the purpose of enabling the photographing of that part of the body being examined. Typically, if the endoscope does not include a photometric optical system for regulating the light intensity, the operator must use a camera which is provided with an automatic exposure control mechanism. In such cases, the kinds of cameras that may be used are limited because they must be provided with a photographic lens suitable for the purpose described and with an automatic exposure control mechanism. Thus, it has been extremely difficult to take a suitable photograph having the desired image magnification.
It is also known to provide, in an endoscopes of the type generally described above, a photometric device which responds to the image light intensity and controls the intensity of illumination of the flashing or illuminating lamp, whichever the endoscope is designed to use during photography. Endoscopes including a latter feature are not limited to use with automatic exposure cameras, but such endoscopes have other disadvantages. Typically, the photometric apparatus includes a beam splitting element, such as a semitransparent mirror, arranged to send a portion of the image light through the viewing lens and another portion of the image light in a direction to impinge upon the photosensitive element used in the photometric control circuitry. As a result of diverging a portion of the light to control the photosensitive element, the remaining light available for image recording is therefore reduced by 20% or more. While it would be possible to increase the amount of light available for image recording by increasing the total amount of illuminating light, the latter increase can adversely affect certain parts of the body, such as mucous membrane areas, which are being examined and photographed. Therefore, the latter expedient is not desirable. Additionally, the problem is even greater in small diameter endoscopes, such as a broncho-fiberscope which is normally only capable of transmitting an extremely small amount of light. In a latter type of endoscopes, a reduction of the quantitative light available for impinging on the film during photography by 20% or more is simply too great to be permitted.